The RAC's annual Cost of Motoring index has revealed the average cost of keeping a car on the road to be a staggering £6,689 per annum, a rise of 14% (£819) on last year. That's an average of £128.64 per week and 55.74 pence per mile, but however you dress it up it is surely a hefty blow to Britain's already cash-strapped motorists.